Earth satellites are widely used for a variety of communication, surveillance, and broadcasting use, and for navigational aid. Certain of these uses require spacecraft attitude stability. For example, a communication or surveillance satellite may be required to point an antenna or camera at a specific location on the Earth's surface. Such pointing is facilitated if the attitude of the spacecraft is well controlled.
Attitude control systems are well known in the art. Such control systems may sense attitude by the use of horizon sensors, sun sensors and the like. An attitude control system using a polar star sensor is described in U.S. patent application Ser. No. 07/511,169 filed Apr. 19, 1990 in the name of Paluszek. Such attitude control system determine the difference between the current attitude and the desired attitude of the spacecraft, and operate torquers for torquing the spacecraft toward the desired attitude. The torquers may include chemical thrusters, momentum or reaction wheels, and/or magnetic coils.
In general, it is desirable for the spacecraft to maintain the desired attitude without resort to the operation of the torquers. The operation of chemical-fuel torquers reduces the available supply of fuel which may be otherwise used for North-South stationkeeping. Momentum or reaction wheels may gradually increase their momenta, eventually requiring the use of chemical or other torquers to reduce the momenta. In any case, the wheel-type torquers and magnetic coils use electrical power, which may be in short supply. In order to reduce the interaction of spacecraft with the Earth's magnetic field, a known technique demagnetizes the spacecraft by probing its fields with magnetic sensors during manufacture for sensing magnetized regions, followed by the application of small magnets to the structure to counteract inherent magnetic fields. Many such small magnets may be used, the weight of which may undesirably reduce the payload of the spacecraft. Even if the weight should be small, it is difficult to demagnetize the spacecraft in this manner, and the magnetic characteristics of the spacecraft may in any case change following manufacture, due to the effects of transporting the satellite into its desired orbit.
A method for reducing the residual magnetic dipole of a spacecraft in orbit can reduce the attitude errors occasioned by passage of the spacecraft through Earth's varying magnetic field, thereby increasing the pointing accuracy and decreasing the need to operate torquers to maintain proper attitude.